1. Technical Field of the Disclosure
The present invention relates to a filter media (a filter material) for an air filter. More specifically, the present invention relates to a filter media for an air filter used for a clean room and a clean bench, which are used in the field of semiconductors, liquid crystals, foods, pharmaceuticals, medical care, or the like, and an air filter provided with the same.
2. Description of the Related Art
In order to collect a dust particle having a submicron to micron unit in the air, a collecting technique of an air filter has been used. Air filters are classified by the difference of the particle diameter to be subjected, or the collecting efficiency, roughly into a coarse dust filter, a middle performance filter, a high performance filter (HEPA filter, and ULPA filter), and the like.
As the major properties required for a filter media for an air filter, there is a pressure drop showing airflow resistance of a filter media, in addition to the collecting efficiency. In order to increase the collecting efficiency of a filter media, it is required to increase the formulation of a fine fiber having a large surface area, however, the pressure drop of the filter media also increases at the same time. The high pressure drop increases the operation load of an intake fan, and causes the increase in the power consumption, therefore, is not preferred from the viewpoint of both of the energy saving and the reduction of running cost. Accordingly, a filter media for an air filter having both of the low pressure drop, and the high collecting efficiency is required. As an index value of the level of low pressure drop and high collecting efficiency of a filter media for an air filter, there is a PF value defined by an equation of Mathematical Equation 1. The high PF value shows that the filter media for an air filter has a low pressure drop and high collecting efficiency. Further, the penetration is obtained by the following equation:Penetration [%]=100−collecting efficiency [%].
                              PF          ⁢                                          ⁢          value                =                                                            log                10                            ⁡                              (                                                      Penetration                    ⁡                                          [                      %                      ]                                                        /                  100                                )                                                    Pressure              ⁢                                                          ⁢                                                loss                  ⁢                                                                          [                  %                  ]                                /                9.81                                              ×                      (                          -              100                        )                                              [                  Mathematical          ⁢                                          ⁢          Equation          ⁢                                          ⁢          1                ]            
As a method for improving the PF value of a filter media for an air filter, of which the main fiber is a glass fiber, a method for allowing a fluorine-containing resin to adhere onto a surface of a fiber (for example, see Japanese Patent Application Publication No. H06-15126), a method for allowing a binder and a silicon resin to adhere to a fiber constituting filter paper (for example, see Japanese Patent Application Laid-Open (JP-A) No. H02-41499), and a method for allowing a binder, a fluorine resin, and a silicon resin to adhere to a fiber constituting filter paper (for example, see JP-A No. H02-175997) have been proposed.
Further, the present inventors have proposed the following techniques. (1) A method for allowing a binder, and a fluorine-based surfactant having the lowest surface tension of 20 dyne/cm or less at the time of adding into pure water at 25° C. to adhere to a glass fiber constituting a filter media (for example, see JP-A No. H10-156116), (2) a method for allowing a binder, and an ether type nonionic surfactant to adhere to a glass fiber constituting a filter media (for example, see JP-A No. 2006-167491), (3) a method for applying a binder liquid containing a cationic surfactant that is a quarternary ammonium salt to a filter media (for example, see JP-A No. 2010-94580), and (4) a method for allowing a binder, and an acetylene-based surfactant to adhere to a glass fiber constituting a filter media (for example, JP-A No. 2003-71219). It has shown by using these methods that the PF value of a filter media for an air filter can be increased by the prevention of clogging of pores due to a binder film.